Email shield works on XP/Outlook 2007 machine but not on 7/Outlook 2010 machine

My Avast e-mail shield appears to be working properly on my Windows XP/Outlook 2007 machine, but not on my Windows 7/Outlook 2010 machine. Both machines are connected to the same e-mail server and receive identical e-mails. Both machines have identical versions of Avast free (V7), with identical configurations.

The XP/Outlook 2007 machine detects trojans on incoming junk mail and properly quarantines them to the virus chest. The same e-mails come in undetected for trojans on my Windows 7/Outlook 2010 machine (my e-mail shield is turned on).

Thoughts?